1. Field of the Invention
This invention relates to field testers. More specifically, this invention includes a field tester of type wherein the inside surface of a projection hemisphere provides for visual field intensity testing with a movable spot of light having examiner selected parameters, such as size, intensity, and brightness, that are maintained constant during a testing sequence in spite of continuously changing focal length and distance from an image projection point.
2. Description of the Prior Art
Field testers are known. It is common to include a hemispheric projection surface and place the tested eye of the patient at the center of the hemisphere. Thereafter, light sources are sequentially flashed on the surface of the hemisphere. After instructing the patient to a central fixation, an examiner notes the ability of the patient to see the sides of the sphere and the light sources flashed. The patient's so called "field" is thereafter recorded and used to plot the absence, presence, and even progress of disease.
These instruments are of several types. In a first type, an array of lights is mounted to penetrate through the surface of the sphere (see for example Gains, U.S. Pat. No. 3,288,546). Such devices are usually limited in the color and intensity of light projected as by the type of light source used. For example, where diodes are used, only monochromatic tests are possible.
In another type of device, light projection is made to coincide as closely as possible to the center of the sphere where the patient's eye and forehead frequently reside. As a consequence, the point of beam projection physically interferes with the point of patient placement. Thus, all the difficulties of off-axis projection are present, including improper focus of a light beam on the inside projection of the dome and variation in intensity of the projected light beam. (See for example Krahn U.S. Pat. No. 4,045,130).
Off-center projection has heretofore been used. Such projection has included scanning the inside surface of the hemisphere with a target path comprising a plurality of semi-closed loops. Generally, the path of the loops has been directed to all points on the inside surface that are equidistant from a projection point. The image is projected in a great circle fashion such that sectors of the inside surface are defined. When one sector has been tested, the projector is moved and another sector is tested. The projector is typically moved in one of two ways: it is swung by a mechanical arm horizontally and transverse of the inside surface in an arc, or it is moved about the periphery of the inside surface. These approaches, although maintaining constant image size and intensity due to the equal distance from the projection point to the inside surface for all points in a sector, are not ideal in a testing environment. The device of the type having a mechanical arm is not suited for automated testing; the arm must swing behind the patient's head during the test, creating a hazard wherein the patient could bump into the swinging area. The device having peripheral projection requires complex rotary movement of its entire optical system. Such an approach unnecessarily complicates the field tester mechanism.